How the Horsepower Calculator Works
Torque is the twisting force of an engine, and horsepower is how fast the engine can do work. Torque alone does not tell you the power, because the same torque at a higher engine speed does more work each second. The Horsepower Calculator joins the two with the engine speed (RPM), so you can find the power from the torque, or the torque from the power.
The formula
horsepower = torque (lb-ft) x RPM / 5252 torque (lb-ft) = horsepower x 5252 / RPM
The number 5252 is not random. One horsepower is 33,000 foot-pounds of work each minute, and one turn of the engine is 2 x pi radians, so 33,000 / (2 x pi) = 5252.1. The calculator uses the exact value, and it works in SI units inside: power in watts = torque in N·m x 2 x pi x RPM / 60, and one horsepower is 745.7 watts.
What you enter
- What to find: the power or the torque.
- The torque in pound-feet (lb-ft) or newton-meters (N·m), or the power in horsepower, metric horsepower or kilowatts.
- The engine speed in revolutions per minute (RPM).
- The drivetrain loss (optional), as a percent, to find the power at the wheels.
Worked example: 300 lb-ft at 5,500 RPM
| Step | Result |
|---|---|
| Horsepower (300 x 5500 / 5252.1) | 314.2 hp |
| Metric horsepower (PS) | 318.5 PS |
| Kilowatts | 234.3 kW |
| Torque in newton-meters (300 x 1.3558) | 406.7 N·m |
| Power at the wheels with a 15% loss | 267 hp (199.1 kW) |
Another example: an engine with 400 N·m at 6,000 RPM makes 400 x 2 x pi x 6000 / 60 = 251,327 W, which is 251.3 kW or 337 hp.
Chart: the power at other engine speeds
If the torque stays at 300 lb-ft, the power goes up in a straight line with the RPM. Each block is about 9.4 hp. Yours is green.
2,200 rpm █████████████ 125.7 hp 3,300 rpm ████████████████████ 188.5 hp 4,400 rpm ███████████████████████████ 251.3 hp 5,500 rpm █████████████████████████████████ 314.2 hp 6,600 rpm ████████████████████████████████████████ 377.0 hp
A real engine does not keep the same torque at all speeds. The torque rises to a peak and then falls, so use the torque and the RPM from the same point on a dyno chart.
Chart: the torque needed for 300 hp
The other way round, a slower engine needs much more torque to make the same power. Each block is about 19 lb-ft.
2,101 rpm ████████████████████████████████████████ 750 lb-ft 3,151 rpm ███████████████████████████ 500 lb-ft 4,202 rpm ████████████████████ 375 lb-ft 5,252 rpm ████████████████ 300 lb-ft 6,302 rpm █████████████ 250 lb-ft
This is why a truck diesel that turns slowly needs a lot of torque, and a small high-revving engine can make the same power with much less.
Why 5,252 RPM is special
At 5,252 RPM, the formula gives horsepower = torque x 5252 / 5252, so the horsepower and the torque in lb-ft are the same number. On a dyno chart in these units, the two lines always cross at 5,252 RPM. Below this speed the torque number is bigger, and above it the horsepower number is bigger. This is not a special property of an engine. It only comes from the units.
Units of power
| Unit | Watts | Notes |
|---|---|---|
| 1 horsepower (hp, mechanical) | 745.7 W | The unit used in the United States |
| 1 metric horsepower (PS, CV, cv) | 735.5 W | Used in Europe and Japan |
| 1 kilowatt (kW) | 1,000 W | The SI unit. 1 kW = 1.341 hp |
- Unit: 1 horsepower (hp, mechanical)
Watts: 745.7 W
Notes: The unit used in the United States - Unit: 1 metric horsepower (PS, CV, cv)
Watts: 735.5 W
Notes: Used in Europe and Japan - Unit: 1 kilowatt (kW)
Watts: 1,000 W
Notes: The SI unit. 1 kW = 1.341 hp
For 300 hp, the same power is 304.2 PS or 223.7 kW. Each block of the chart is about 7.6 units.
hp ███████████████████████████████████████ 300 PS ████████████████████████████████████████ 304.2 kW █████████████████████████████ 223.7
Power at the wheels
The power of an engine is measured at the crankshaft. Some of it is lost in the gearbox, the driveshaft and the axles before it reaches the wheels. A typical loss is about 10% to 15% for a manual gearbox, 15% to 20% for an automatic, and more for an all-wheel drive. If you enter a loss, the calculator shows the power that is left at the wheels.
Tips
- Use matching numbers. The torque and the RPM must be from the same moment, for example the value at the peak of the power.
- Check the unit. A torque in N·m is 1.3558 times a torque in lb-ft. Pick the unit in the calculator, and it makes the change for you.
- Compare like with like. Some makers give the power in PS or kW, and others in hp, so a car with 300 PS has about 296 hp.
What this calculator does not include
- The power of an engine from its size, its fuel or its turbo. It needs a measured torque or power.
- Corrections for air temperature, pressure and height.
- The power of electric motors at a set current and voltage (use the power in kilowatts).
This tool gives an estimate for general information. For a real engine, use a dyno test.